The present invention relates to a process for the manufacture of a high purity and high concentration aqueous solutions of hydroxylamine. Hydroxylamine free base (hereinafter H2NOH, HA or HAFB) is a specialty chemical widely used in the pharmaceutical, agricultural chemical and electronic industries. Potential applications of HA-related chemicals are also possible in automotive, aerospace industries and as an oxidant for various formulations in liquid propellants.
Since its introduction by Nissin Chemical Co., Ltd. of Japan in the early 1970, the commercial product has been shipped as a 50% aqueous solution for more than 30 years now. The rapid growth of the use of HA in the semiconductor industry accounts for the increase of the market demand in the recent years. A double-digit growth in 2003 has been projected by a marketing study. In addition, a successful commercialization of the HA-related liquid propellants for air bags and a monopropellant thruster will, for sure, increases the demand in the market further.
There are three basic manufacturing technologies that have been disclosed in the patent literature:                Reaction of a base with a hydroxylamine salt (neutralization process);        Ion-exchange process;        Electrochemical process.        
In the neutralization process, once the free base is liberated, the process involves additional steps to achieve a 50% concentration product and with extremely high purity. These additional process steps include:                Separation and removal of side products;        Concentration of the hydroxylamine to a desired concentration level; and        Purification of the product to a purity required for the intended applications.        
Depending on the applications in different industries, the desired and sometimes required metal impurity levels, especially those of transition metals such as iron, cobalt, chromium, can vary from hundreds of ppm to several ppb. Furthermore, the stability of a hydroxylamine product depends on temperature, hydroxylamine concentration, metal impurity levels and other factors. Therefore, the concentration and purification process chosen for producing the desired product specification is extremely critical.
Many attempts have been disclosed by various inventors for the preparation of hydroxylamine using hydroxylamine salts with various bases.
For example, DE-A-3528463 discloses the neutralization with calcium, strontium, or barium hydroxide. The removal of finely divided alkaline earth metal sulfate side products presents considerable difficulties. In addition, calcium sulfate side product has a relatively high solubility, thus it cannot be totally removed by filtration. Strontium hydroxide and barium hydroxide are highly priced and known to be quite toxic.
U.S. Pat. No. 5,472,679 describes a process for the preparation of hydroxylamine by reacting a hydroxylamine sulfate (HAS) solution with a suitable base at up to 60° C. The resulting mixture is distilled to dryness under reduced pressure at <65° C. During the distillation, the metal impurity concentration increases along with the HA concentration, thus risking the danger of explosion.
U.S. Pat. No. 4,956,168, DE-A-1247282, and EP-A-108294 disclose a process in which alcoholic solutions of free hydroxylamine are obtained by reacting hydroxylammonium sulfate with ammonia in an alcohol solvent followed by removing the precipitated ammonium sulfate side product. However, owing to the flammability of the alcohol solutions and the high expense in recovery of the alcohol solvents, this process is difficult to be commercialized at a large scale.
DE-A-3601803 describes the use of lower alcohol solvents. The precipitated ammonium sulfate side product is separated, water is added and the alcohol is distilled off from the solution. Again, the flammability of the lower alcohol solvents and the instability of the hydroxylamine prohibit the industrial application of the process.
WO97/22551 discloses a process by which a solution, resulting from neutralization of HAS with a suitable base, is separated into an aqueous hydroxylamine fraction and a salt solution by distillation at a temperature above 80° C. (stripping). The HA fraction is further concentrated in a distillation column. With a strong base such as sodium hydroxide, a complete neutralization is obtained. However, the ready decomposition of HA, its sensitizing effect, and the tendency of sodium sulfate to cake make the process difficult to practice industrially.
The use of aqueous ammonium hydroxide as a base would have two major advantages over sodium hydroxide: i) lower cost, and ii) no metal contamination. However, ammonium hydroxide is a relatively weak base, only 60-70% conversion is observed under the above stripping procedure. Therefore, the use of ammonium hydroxide is not viable with the stripping method of WO 97/22551.
U.S. Pat. No. 6,299,734 B1 discloses a process by which the hydroxylammonium salt in an aqueous phase is treated with ammonia by a countercurrent method and at the same time the solution obtained is separated into an aqueous hydroxylamine solution and a salt fraction by stripping with steam in a stripping/reaction column. In a particularly preferred embodiment, the neutralization of hydroxylamine salt with ammonia is carried out by the countercurrent method and the stripping of the hydroxylamine from the salt solution is effected in combination with partial concentration of the hydroxylamine solution in only one column, i.e. a stripping/reaction/distillation column. The process improves over the simple stripping process by increasing the yield of hydroxylamine from about 60% to 90%. However, the stripping/reaction/distillation process employs substantial amount of steam. Not only the energy cost on the steam used is high, additional energy is also required to concentrate the hydroxylamine product because of low hydroxylamine concentrations produced from the process to avoid the hydroxylamine decomposition when exposed to >80° C. temperature during the manufacturing process.